At your 12 week scan they will check for baby's heartbeat, they will have a look at the nuchal fold measurement and CRL. They send these measurements off with your blood sample to the lab who analyses them alongside your booking information such as Diabetic status, smoking status, BMI and it will return an individual risk factor relating to your baby for Trisomy 21, 13 & 18. This isn't a diagnosis just an indicator.
If you came back in the low risk group you wouldn't be offered any further testing, if you came back a high risk (I think the threshold is 1:150) then you would be offered an NIPT (more accurate blood test which comes back high or low change) and/or an invasive diagnostic test.
Generally at 12 weeks they can't see fetal abnormalities, your 20 week scan (FNS) is a fetal anomaly scan where they evaluate the anatomic structures of the fetus, placenta, and maternal pelvic organs and when any suspected abnormalities would usually be identified.
Your 12 week scan is very basic as baby is so small still.
We announced our pregnancy at 14 weeks once our screening results had come back, but I was still mindful that it wasn't until 20 weeks that baby's anatomy was checked for abnormalities.
You have to just weigh up when the time is right for you to tell people.